Iron Tail is a Level 41 Elite Monster located South-East of Wind's End in The Great Desert. He can be found sleeping under a large tree. (according to the video I just watched if you get as far as the vampire witches you have gone too far, the arrow on the map here is a little too far east).
He is one of the mobs required for the Achievement - Rare Blood in The Great Desert.
His respawn timer is 4 hours.